When a hydraulic system failure occurs or when the engine of the prime mover is not running to drive the pump for a brake system having spring applied-pressure released brakes, the brakes are spring applied by the loss of hydraulic pressure. When such a loss of pressure occurs, a vehicle of this type cannot be towed to a suitable repair station until the brakes are again pressurized, thus releasing the brakes.
Manual pumps typically are used to pressurize and release the brakes. These known manual pumps typically include at least three ports: a pressure port, a brake port, and a tank port. In these known manual pumps, when one is manually pumping to pressurize the brakes, the pressure port is typically blocked and hydraulic fluid is drawn from the tank to pressurize the brake. One known manual pump includes more than one rod, i.e. a first rod that acts as a piston for the manual pump and a second rod that acts as spool of a valve to block flow from the pressure port to the brake port. Another known manual pump includes valves in combination with rods. The valves require the operator to turn or adjust the valves to block flow from the pressure port to the brake port. Both of these known manual pumps require a complex manifold structure to provide the proper fluid communication between the ports in different operating modes and also require operator training to know which rods to push or which valves to turn.
Many vehicles that employ spring applied-pressure released brakes also employ hydraulic motors to drive the vehicle. When the engine is not running to drive the pump for the hydraulic motors, typically a shut-off valve is disposed in the hydraulic circuit to block the flow of hydraulic fluid through the hydraulic motors thus prohibiting rotation of the hydraulic motor and thus the vehicle. These hydraulic motors can also be used to perform dynamic braking to stop the vehicle. Dynamic braking is also performed by blocking flow through the motors so that the motors cannot rotate. Release valves or counter-balance valves can be disposed in the circuit to dump the flow of hydraulic fluid during dynamic braking so that the hydraulic motors do not come to an abrupt halt.
As stated above, when the engine is not running or a hydraulic system failure has occurred the flow of hydraulic fluid through the motors stops and, therefore, blocks, or greatly inhibits, rotation of the hydraulic motors. When the vehicle that includes these hydraulic motors needs to be moved, the shut-off valve can be short circuited to allow for the movement of fluid through the hydraulic motors. If the motors are not properly short circuited, then the motors can act as pumps that deliver pressure downstream to other components, e.g. spring applied-pressure released brakes, of the hydraulic circuit. Typically, a needle valve is located in the hydraulic circuit remote from the aforementioned manual pump used to pressurize the brakes. To move a vehicle that includes spring applied-pressure released brakes and hydraulic motors that are blocked upon loss of pressure, one must pressurize the brakes using a manual pump and move to another location on the vehicle to open a needle valve to short circuit the shut-off valve that blocks flow through the motors.